1.—Field of the Invention
The abovementioned invention concerns the selective transfer of one or several elements we can call labels, from a manufacturing support to a receiving support.
It concerns, in particular, the transfer of semi-conducting chips, partially or fully completed, from their initial substrate on which they are manufactured to a new substrate (or receiving support) that can itself be treated by microelectronic techniques.
The invention enables us, in particular, to transfer chips that may have been electronically tested, for example, chips of 1 mm2 to 1 cm2 area, from their initial substrate to a support made of treated or non-treated semi-conducting material, transparent material, (example: glass), a soft or rigid support (example: plastic) or a support made of ceramics. It also enables us, for example, to transfer optoelectronic components such as a vertical cavity surface emitting lasers or VCSEL or small pieces of III-V semi-conductor from their initial substrate on silicon small plates that can be prepared according to microelectronic techniques in order to obtain III-V semi-conductor elements on silicon.
It can also apply to the transfer of electronic circuits, electronically tested and thinned down if necessary, on a plastic card by means of glue to make smart cards.
2.—Description of the Prior Art
There exist several methods of transferring semi-conducting chips or group of chips to a receiving support. We can mention the technique called “epitaxial lift off”, organic or mineral gluing (by molecular adhesion) and the technique called “flip-chip”. This latter method does not enable us to handle individual components of small dimension or extreme thinness as with the two previous ones.
The technique of mineral gluing or molecular adhesion is well known for numerous materials. Molecular adhesion includes two different types of gluing: absorbent gluing and hydrophobic gluing. In the case of absorbent gluing, the gluing results from the evolution of the interaction of —OH groupings on the surface of a structure toward the formation of Si—O—Si bonds in the case of silicon oxide. The forces associated with this type of, interaction are strong. The gluing energy, in the order of 100 mJ/m2 at room temperature, reaches 500 mJ/m2 after annealing at 400° C. for 30 minutes. In the case of hydrophobic gluing, the gluing results from the evolution of the interaction of the —H or —F groupings on the surface of the structure towards the Si—Si bonds in the case of silicon adhesion. The gluing energy is weaker than for absorbent gluing up to temperatures in the order of 500-600° C. The gluing energy is generally determined by the blade method divulged by W. P. Maszara and al., in the article “Bonding of silicon wafers for silicon-on-insulator” published in J. Appl. Phys. 64(10), Nov. 15, 1988, pages 4943 to 4950.
The control of the gluing energies can enable us to achieve reversible molecular adhesions where, as divulged in the FR-A-2 781 925 document, the adhesion energy of the elements can be low on the transfer support and the elements to be transferred can be treated to have a stronger adhesion energy on the receiving support than on the transfer support, thus enabling us to selectively transfer an element among n. In that document, the adhesion energy is lower between the elements to be transferred and the transfer support than between the elements and the receiving support. Yet, the energy we obtain during the gluing between the elements and the receiving support is an adhesion energy at room temperature between 0 and 200 mJ/m2 depending on the surface treatment and the materials used. The bonding energy between the elements to be transferred and the transfer support is therefore below 200 mJ/m2. The method described in that document does not enable us to transfer elements strongly adhered on the transfer support, for example, at energy above 200 mJ/m2.
FR-A-2 796 491 divulges the transfer on a final substrate, of chips individually handled on a transfer support on which they have been glued beforehand. The chips are individually handled on the transfer support, thanks to openings made through that transfer support and going through it completely. A mechanical (for example, by means of an awl), chemical or pneumatic action, or a combination, enables us to disconnect a chip from the transfer support and to have it adhered on the receiving support. This approach can apply to the low adhesion of a chip on the transfer support and to a strong one on the receiving support. It requires the preparation of a special transfer support that has been conditioned to enable the selection of each element to be transferred. In addition, it is not compatible with the standard micro-electronic equipment such as the “pick and place” equipment.
The U.S. Pat. No. 6,027,958 and WO-A-98/02921 documents divulge the transfer of fine elements processed on SOI substrate for the purpose of increasing the integration density and making the components less fragile. At first, the components are made by means of microelectronic classical technologies; then they are linked together by a metallic deposit. According to that method, the elements to be transferred are adhered on a transfer support on which a chemical stoppage layer has been deposited by an adhesive material. The initial substrate is taken out to uncover the stoppage layer. Next, the components are collectively joined on a receiving support. The transfer support is then taken out.
This method enables us to transfer a set of components in thin layers toward a receiving support. It is therefore a collective process. In that transfer method, the components are kept together with a thick support that enables us to handle them. Never can an individual component be separated from the others due to the thick substrates (initial substrate, transfer support, receiving support) that are always in contact with the components. This method does not allow the selective transfer of an element in thin layer.
The technique called “epitaxial lift-off” enables us to transfer a component in an individual manner. The first step consists in making by epitaxy, components to be transferred (see American patent No. U.S. Pat. No. 4,883,561 in the case of a simple transfer). The epitaxial layers can be inverted (see the American patent No. U.S. Pat. No. 5,401,983 in the case of double transfer). Before the formation of all the layers making up the component, a sacrificial layer is epitaxed on the growth substrate. The labels are defined by photolithography. A resin is deposited on the labels so as to curve them and give them a concave structure for the renewal of the under-engraving and the evacuation of the attacking residues. The sacrificial layer is then chemically attacked by wet process. Since the film formed by the epitaxed layers is curved by the resin, the film bends down little by little, which enables the attacking solution to penetrate the cracks in formation between the film and the substrate. When the attack is over, the film can be recovered by means of vacuum pliers and then transferred on a target substrate (see the American patent No. U.S. Pat. No. 4,883,561) or on a transfer support (see the American patent No. U.S. Pat. No. 5,401,983). After the transfer, the resin is chemically removed; then the labels, transferred on the receiving substrate, are submitted to thermal treatment under pressure to remove the gas trapped in the interface.
In that technique, the sacrificed layer is an epitaxed layer; hence the name “epitaxial lift-off”. The elements can be individually or collectively transferred on a transfer support and be made interdependent by Van der Waals forces on a receiving support, then re-fired once the transfer support has been removed. The main advantage of this technique is that it recovers the initial substrate. However, due to the lateral under-engraving of the sacrificial layer, the dimensions of the chips to be transferred are limited. The article of E. YABLONOVITCH and al., Appl. Phys. Lett. 56(24), 1990, page 2419 mentions a maximal size of 2 cm×4 cm and quite long attacking times. The maximum speed is 0.3 mm/h in the case of low lateral dimensions (below 1 cm). Furthermore, since that attacking speed depends on the curve of the elements to be transferred for the discharge of the attacking residues, it restricts the thickness of the transferable components to 4.5 μm (see the article of K. H. CALHOUN and al., IEEE photon. Technol. Lett., February 1993).
The “lift-off” technique has been known for many years (see, for example, the American patent No. U.S. Pat. No. 3,943,003). This technique includes depositing a thick photosensitive resin on a substrate, exposing and opening that resin at places we wish to deposit a material, which, generally, is a metal. The material is deposited by a cold depositing method (generally by pulverization) on the whole surface of the substrate. The material deposited really only adheres at the places where the resin has been opened. The resin is then dissolved in a solvent (for example, acetone) and only the adhering parts of the material remain.
Other methods are proposed in the document EP-A-1 041 620. They enable us to transfer thin chips on a host support. These methods are based on the individual treatment of the chips. The chips are glued one by one on a transfer support, are individually made thinner and are then transferred on a host support. It is not a collective treatment for the thinning, followed by an individual transfer from the substrate.